feat(nfs): centralize nixdesk jellyfin nfs client module

Move the nixdesk jellyfin NFS client configuration from individual
host services to a centralized NixOS module in `modules/system/nfs/`.

This change:
- Creates `modules/system/nfs/default.nix` containing the `nixdeskJellyfin`
  flake module.
- Removes redundant NFS client definitions from `modules/hosts/nix-server/`
  and `modules/hosts/r5500/`.
- Updates `modules/hosts/r5500/configuration.nix` to use the new
  centralized module.
- Cleans up unused local disk configuration in `modules/hosts/14900k/`.

modules/hosts/nix-server/_services/nixdesk-nfs-client.nix

@@ -1,35 +0,0 @@
-# NFS mounts of nixdesk (14900k) bulk storage for nix-server. Exports live in
-#   modules/hosts/14900k/_private/jellyfin-nfs-export.nix
-#
-# Jellyfin library paths (see also services/jellyfin.nix):
-#   Movies → /mnt/nixdesk-jellyfin/movies
-#   Shows  → /mnt/nixdesk-jellyfin/tv
-#
-# If you see "Stale file handle" under /mnt after changing exports or fsid on nixdesk, drop the
-# old client mount and let automount reattach, e.g.:
-#   sudo umount -l /mnt/nixdesk-jellyfin
-#   ls /mnt/nixdesk-jellyfin
-# (or reboot nix-server.)
-{ ... }:
-let
-  nfsExportHost = "192.168.2.25";
-  # nfsvers+tcp: predictable Linux↔Linux; lookupcache=none: fewer stale dentries after export changes.
-  nfsClientOpts = [
-    "rw"
-    "noatime"
-    "nofail"
-    "_netdev"
-    "nfsvers=3"
-    "tcp"
-    "lookupcache=none"
-    "x-systemd.automount"
-    "x-systemd.idle-timeout=3600"
-  ];
-in
-{
-  fileSystems."/mnt/nixdesk-jellyfin" = {
-    device = "${nfsExportHost}:/mnt/deep/jellyfin";
-    fsType = "nfs";
-    options = nfsClientOpts;
-  };
-}